1. Technical Field
This invention relates to a method and apparatus for attenuating acoustic waves (or ring around acoustics) propagating through the walls of a pipe for a clamp-on ultrasonic flow meter.
2. Background Information
Clamp-on ultrasonic flow meters are a desirable tool for determining characteristics of a fluid flow traveling through a pipe. The flow meters typically include a plurality of ultrasonic sensors, each having a transmitter and a receiver. In some sensor configurations, ultrasonic signals emitted from the transmitters travel through the immediate pipe wall, the fluid flow disposed within the pipe, and through the opposite pipe wall where they are sensed by the receiver portion. In other sensor configurations, the transmitters and receivers are disposed on the same side of the pipe; the sensed fluid flow signal component is one that has reflected off of the opposite pipe wall and traversed the fluid flow a second time. Regardless of the sensor configuration, the received signal is processed to determine information such as flow velocity, volumetric flow rate, water cut, etc.
One of the primary challenges associated with clamp-on ultrasonic flow metering is distinguishing the fluid borne signal component from a structural borne component that is generated when the ultrasonic signal travels through the pipe on a path substantially normal to the surface of the pipe. The fluid borne component contains useful information relating to characteristics of the fluid flow. The structural signal component, on the other hand, does not travel through the fluid flow, and therefore does not contain useful information relative to the fluid flow. In fact, the structural borne signal is predominantly a shear wave that travels within the pipe, and can “ring-around” the pipe circumference several times before dissipating. Collectively, the structural borne “ring-around” signals create interference that makes it difficult to extract the desired fluid borne signal component. Hence, there is considerable value in minimizing or eliminating structural borne signal components.